Filters have been proposed for use in anesthesia equipment to filter out bacteria and other microorganisms from inhaled anesthesia gases. One example is U.S. Pat. No. 3,556,097, which shows in FIGS. 11-15 a large filter that is currently on the market. One of the problems with such filter is that it has a very large dead space of 150 to 200 cubic centimeters in the filter excluding the tubular fitments on each end of the filter. With such large dead space, the patient is rebreathing a substantial portion of his exhaled respiratory gases.
It is desirable to have an anesthesia filter with less than 50 cubic centimeters of dead space, excluding tubular adapters. However, in the past, such small filters have had a very high pressure drop of 3 inches of water at 12.5 L gas per minute flow rate. Such high pressure drop limited the use of such a filter to inhalation therapy use where a pump or other mechanism forced air into the patient's lungs and then extracted it by vacuum. It was not suited for anesthesia use in which the anesthesia gases are transferred across the filter by normal breathing of the patient.
Even with such high pressure drop filters, as shown in U.S. Pat. No. 3,815,754, there was a difficult manufacturing procedure to completely seal the filter within the rectangular housing. Separate end plates (30 and 31 of FIG. 1) were required to seal the serpentine edges of the pleated filter.